You've made changes that may require Jira to be re-indexed (this message is a little too frequent - it appears after some changes that do NOT need a re-index, but it's being cautious)
It is saying what you need to do - re-index Jira.
The index is a cached copy of some of the data in the database, optimised for searching - this srops Jira having to read the database for every single filter/search/report your users run. What you've just changed has invalidated some of that data and it needs to be rebuilt.
It's not critical, just something you should do as soon as you can. Your users may find searches don't work properly until you've done it, but your core data is safe.
just to add some additional information to "Nic Brough"s answer,
it's possible to index the system in the background while your users still have access to jira. This would take longer than rebuilding the index while locking jira down.
From my experience it's more practical to use the rebuild index function but that's just my preference since i have experience some freezing issues in jira after using the background indexing function.
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